Electrolyte and method for electrodepositing a pink gold alloy



United States Patent 3,380,898 ELECTROLYTE AND METHOD FOR ELECTRO-DEPOSITING A PINK GOLD ALLOY Marius Danemark and Ren Cathrein, Geneva,Switzerland, and Atkin Y. Simonian, Fort Lee, N.J., assignors to Sel-RexCorporation, Nutley, N.J., a corporation of New Jersey No Drawing. FiledJune 18, 1965, Ser. No. 465,201

4 Claims. (Cl. 204-44) This invention relates to an electroplatingprocess and bath for the production of pink gold deposits consistingessentially of a copper-gold-antimony alloy.

Among the objects of the invention is to provide an improvedelectroplating bath and process for the production of bright pink golddeposits.

Acid gold piating baths for the deposition of gold operating at a pH ofbetween 3 and 5 or 6.5 are known. It is also known that bright, hardgold deposits obtained when base metal ions capable of codepositing withthe gold are added to such baths. See, for example, US. 2,905,601.

One of the most difiicult colors to obtain by the acid gold platingprocess is a bright gold deposit with a pink coloration. This isdifiicult because copper is the main metal ion producing pink colorationand with copper ions the bath is sensitive to changes in concentration,etc., so that uniformly-reproducible, copper-gold alloy deposits can beobtained only by careful regulation of the bath components and of theconditions of operation. Also, after a time, a slight haze develops indeposits obtained from such baths, the haze becoming more irnportant asthe bath or solution ages.

Among the objects of this invention is to provide a uniformlyreproducible bright, hard, copper-gold alloy deposit with a pinkishcoloration.

In copending application Ser. No. 465,181, filed on even date herewith,there is disclosed a process of preparing such rose or pinkish gold byproviding an acid type of gold alloy electroplating bath having thefollowing ingredients and conditions of operation:

The copper complex is a copper salt of any suitable chelating orcomplexing agent having a negative logarithm of stability constant forcopper of about 17 or less. Such complexing agents includeethylenediamine tetracetic acid (EDTA), diethylenetriamino pentaaceticacid, N-hydroxyethylethylenediarnine triacetic acids or the sodium orother alkali metal salts thereof. The excess chelating agent is the sametype of compound.

Specular bright deposits are obtained from such baths at thicknesses of25 microns and over so that the gold layer is thick enough to providecorrosion resistance as well as the pink decoration.

The weak acid is one, as defined in the literature, which has a pK valueof over about 3 and such weak acids include the organic acids andphosphoric acid. Where the acid employed .is polybasic, the partially3,380,898 Patented Apr. 30, 1968 neutralized acid salts or mixturesthereof may be employed in place of the acid and salt. Thus KH PO orcorresponding salt of sodium, lithium or ammonium, may be employed inthe above bath as the weak acid and salt. Similarly the mono-potassiumsalt of citric acid may be employed as the acid and salt combination.

Although it is preferred to add the antimony as the tartrate, it canalso be added as other salts which are soluble in the bath.

Although copper is an essential alloying metal for obtaining a pinkcoloration in alkali gold cyanide baths, and is sufiicient by itself forthe production of pink coloration in such baths, the antimony, asdisclosed in this specification, is also essential for the production ofpink coloration and specular effects with the acid gold baths disclosedherein. The alloy deposits obtained from baths as described aboveconsists essentially of to gold, 14 to 24% copper and 0.5 to 1.5%antimony. The alloy has a Knoop hardness of about 230-280 kg./mm. with a25 g. load.

The present invention is based on the discovery that minute additions oflead to such baths decrease a tendency which such baths have to formhaze when the agitation and current density are not controlled properly.Thus the bath contains the ingredients sets forth above and in additionthe following:

Lead (added as soluble lead salt as lead acetate) 15-200 mg./l.preferably 15 mg./l.

The exact mechanism of this phenomenon is not understood, but in view ofthe small amounts needed, the effect would appear to bedifiusion-controlled. It is preferred to add the lead as the acetate, acommon water soluble lead salt, but any lead compound soluble in thebath, such as lead ethylenediamine-tetra-acetate, may be used.

The following example is given to illustrate in detail a preferred bathand method of operating the same. It is to be understood that thespecific details given in the example is not to be considered aslimiting the scope of the invention.

EXAMPLE Grams/liters Gold [as KAu(CN) as metal 3 Copper (as salt of NaEDTA), as metal 4 Antimony tartrate, as metal 3 Na EDTA 20 KH PO 60 Lead[as Pb(C H O* 63%], as metal .015

pH, adjusted to 5.0.

Brass panels plated to a thickness of 25 microns with the bath at atemperature of 50 C. and at a density of about 10 amp./drn. (amperes persquare decimeter) are full bright and pink in color.

The lead content was increased to 200 mg./l., the only eifect being anincrease in brightness and levelling action.

Substitution of tetrasodium polyphosphate or the potassium salt ofpolyphosphate for the orthophosphate gave similar results.

Baths made up with citric acid neutralized to a pH of 5.0 gave resultsslightly less satisfactory than the inorganic acids, but the panels soplated were quite attractive. Baths prepared from tartaric acid diddevelop a precipitation due the formation of potassium bitartrate whenpotassium gold cyanide was used. Acetic acid-potassium acetate tended toprecipitate out some of the antimony but gave satisfactory deposits.

The features and principles underlying the invention described above inconnection with specific exe-mplifioations will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims shall not be limited to any specific feature ordetails thereof.

We claim:

1. Process for electrodepositing bright, hard gold-copper alloys havinga pink coloration Which comprises electrolyzing an aqueous solutioncomprising the following ingredients:

maintaining said bath at a temperature of 40 to 60 C. and regulating thecurrent density to a value of 0.5 to 2.0 amp./-dm.

2. The process as set forth in claim 1 wherein the bath contains thefollowing ingredients:

Gold (added as alkali metal gold cyaanide) g./l about 4.8 Copper salt ofNa EDTA (calc. as

Cu) g./l about Antimony tartrate (calc. as Sb) g./l about 1.5 Na EDTAg./l about 1.5 KH PO g./l about 60 Lead (added as lead acetate) m.g/labout said bath being operated at a pH of about 5.3, a temperature ofabout 50 C. and a current density of about 1.5 amp./dm.

3. An aqueous bath for plating hard, specularly bright copper-gold alloyconsisting essentially of the following:

4. An aqueous bath for plating hard, specularly bright copper-gold alloyconsisting essentially of the following:

Gold (added as potassium gold cyanide) g./l about 4 Copper salt ofNagEDTA (calc. as Cu) g./l about 5 Antimony tartrate '(calc. as Sb) g./labout 1.5 Lead (added as lead acetate) mg./l about 15 Na EDTA g./l about20 KH PO g./l about Adjusted to pH of 5.3.

References Cited UNITED STATES PATENTS 2,724,687 11/1955 Spreter et al204-43 2,905,601 9/1959 Rinker et al. 20444 XR 2,967,135 1/1961 OstroWet al 20446 XR 3,020,217 2/ 1962 Rinker 20446 XR OTHER REFERENCES Thews,Edmund R., The Production of Colored Gold Finishes, Metal Finishing, pp.-85, September 1951.

Pritchard, Alfred K., Jr., Deposition of Pink Gold, Metal Industry, vol.31, No. 12, p. 408, 1933.

HOWARD S. W'ILLIAMS, Primary Examiner.

G. KAPLAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,380,898 April 30, 1968 Marius Danemark et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 2, line 53, "10" should read 1.0

Signed and sealed this 11th day of November 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. JR. Attesting Officer Commissioner ofPatents

1. PROCESS FOR ELECTRODEPOSITING BRIGHT, HARD GOLD-COPPER ALLOYS HAVINGA PINK COLORATION WHICH COMPRISES ELECTROLYZING AN AQUEOUS SOLUTIONCOMPRISING THE FOLLOWING INGREDIENTS: